A conventional developing device conveys a developer, which is conveyed by an agitation conveying member such as a screw member and held on a surface of a developer holding unit such as a developing sleeve, to an area facing a latent-image holding unit in accordance with a surface movement of the developer holding unit. Then, a toner included in the developer is transferred onto a latent image formed on the latent-image holding unit, so that the latent image is developed into a toner image. After developing the latent image, residual developer is returned to the agitation conveying member in the developing device in accordance with the surface movement of the developer holding unit. After that, as the developer is conveyed by the agitation conveying member, a toner concentration of the developer is detected by a toner-concentration detecting unit. Based on a result of the detection, the developer is refilled with an appropriate amount of a toner, and then supplied to the developer holding unit again.
Sometimes the amount of the toner included in the developer changes due to fluctuations in the environmental conditions and/or an amount of electric charge on the toner. As a result, a result of the detection by the toner-concentration detecting unit fluctuates even though the toner concentration has not changed which results into an erroneous detection. The erroneous detection can be prevented by pressing the developer firmly at a detecting position where the toner-concentration detecting unit detects the toner concentration so as to have the amount of the toner correspond to the toner concentration. For example, according to a technology disclosed in Japanese Patent Application Laid-open No. H06-308833, as in a graph of FIG. 10, by pressing 30 [g/cm2] of a developer with a pressure of 9.88×300 N/m2 or more, a result of a detection by a permeability sensor as the toner-concentration detecting unit can be kept constant regardless of the charge amount of the toner.